1. Field of the Invention
The present invention relates to a heating apparatus, and more particularly, to a heating apparatus comprising a heating device with a plate-shaped positive temperature coefficient thermistor (a positive temperature coefficient thermistor is referred to as a PTC thermistor hereinafter).
2. Description of the Prior Art
Conventionally, a PTC thermistor having a positive temperature coefficient, i.e., having such a temperature characteristic that the resistance thereof increases with increasing the temperature, has been used as an electronic circuit device, such as a temperature detecting switch which turns on or off responsive to change in the temperature, or a temperature detecting device. Further, the PTC thermistor has widely been used as a heating device for heating a fluid such as air or water.
FIGS. 1 and 2 show prior art heating units 1 and 11 for heating apparatuses for heating a fluid such as air or water, each heating unit comprising PTC thermistors 3.
Referring to FIG. 1, the heating unit 1 comprises the PTC thermistors 3 having electrode films 2 formed on both main surfaces thereof, and a pair of terminal plates 4 are provided on the respective electrode films 2 so as to be in contact with the respective electrode films 2. A pair of spacers 5 are provided between the opposing one portions of both ends of the respective terminal plates 4, wherein each spacer 5 is made of an electrically insulating material such as ceramics or resin, and the terminal plates 4 are fixed on the spacers 5 by metal rivets 6. Then, the PTC thermistors 3 are fixed between a pair of terminal plates 4. Further, lead wires 7 are respectively inserted into crimp-style lead terminals 4a of the respective terminal plates 4, and the lead terminals 4a are pressed so that the lead wires 7 are fixed therein. Thereafter, all surfaces of the heating unit 1 are covered by an electrically insulating tube or film cover (not shown and referred to as an insulating cover hereinafter), made of an electrically insulating material. Finally, a pair of heat radiating plates (not shown) are provided through the insulating cover on the respective terminal plates 4 of the heating unit 1, resulting in a conventional heating apparatus.
Referring to FIG. 2, the heating unit 11 comprises the PTC thermistors 3 inserted between a pair of terminal plates 13 which are fixed in parallel to each other using a pair of spacers 12 of an electrically insulating resin material. Respective projections 14 formed on the spacers 12 are inserted into holes (not shown in FIG. 2) formed in both ends of the respective terminal plates 13, and are deformed so as to fix the pair of terminal plates 13 on the spacers 12. The lead wires 7 are respectively inserted into crimp-style lead terminals 13a of the respective terminal plates 13, and the lead terminals 13a are pressed so that the lead wires 7 are fixed therein. Thereafter, the whole surface of the heating unit 11 is covered by an insulating cover. Finally, a pair of heat radiating plates (not shown) are provided through the insulating cover on the respective terminal plates 13 of the heating unit 11, resulting in another conventional heating apparatus.
However, since the metal rivets 6 are projected from the surfaces of the terminals plates 4 in the heating unit 1 shown in FIG. 1, there are limitations when mounting the heat radiating plate on the heating unit 1, for example, which make it difficult to mount the heating unit 1 in the heating apparatus. Further, there are such problems that the heating unit 1 has a number of parts, and also it is necessary to provide a tool for pressing the lead terminals 4a so as to fix the lead wires 7 therein.
Further, the heating unit 11 shown in FIG. 2 has a smaller number of parts than the heating unit 1 shown in FIG. 1. However, since only a pair of terminal plates 13 is fixed on the spacers 12, the heating unit 11 has an extremely low mechanical durability, and also the structure of the heating unit 11 becomes stable only when covering the whole unit with the insulating cover. Therefore, in the heating unit 11 shown in FIG. 2, it is necessary to cover the heating unit 11 with the insulating cover, and also, the heating unit 11 has an extremely small mechanical durability as described above.